1. Field of the Invention
The present invention relates to a receiver and a reception method for receiving wireless transmission signals. More specifically, the present invention relates to a receiver and a reception method for receiving OFDM (Orthogonal Frequency Division Multiplexing) signals which are transmitted by applying inverse Fast Fourier Transform (FFT) to a plurality of carriers to convert carriers along the frequency-domain into signals along the time-domain.
Still more specifically, the present invention relates to a receiver and a reception method for estimating channel characteristics during OFDM signal decoding and particularly to a receiver and a reception method for performing inverse FFT for ranging and estimating channel characteristics of time response.
2. Description of Related Art
Special attention is paid to wireless Local Area Network (LAN) as a system to free users from cabling of hardwired LANs. The wireless LAN can eliminate most of cables in working spaces such as offices. Accordingly, it is possible to relatively easily move communication terminals such as personal computers (PCs). In recent years, there is a remarkably increasing demand for wireless LAN systems as they achieve higher speed and become available at reduced costs. Recently, introduction of a personal area network (PAN) is especially being considered to construct a small-scale network for information communication between electronic devices available around users. For example, there are provided different wireless communication systems and wireless communication apparatuses using frequency bands such as 2.4 GHz and 5 GHz bands that need not be licensed by governing legal authorities.
In recent years, for example, attention is focused on the “ultra wide band (UWB) communication” as a wireless communication system capable of short-distance, ultrafast transmission. The system performs wireless communication by carrying information on very weak impulse sequences. It is expected to put the system into practical use. Presently in IEEE802.15.3 and the like, there are devised data transmission systems having the packet structure including preambles as access control systems for ultra wide band communication.
If a wireless network is constructed under a working environment where many devices are mixed in a room, it is possible to suppose that a plurality of networks are constructed in an overlapping fashion. A wireless network using a single channel cannot provide any countermeasure against a case where another system interrupts during communication or an interference occurs to degrade the communication quality. To solve this problem, there is proposed a multi-channel communication system that provides a plurality of frequency channels and uses one of the frequency channels to operate. If an interference occurs to degrade the communication quality during communication, for example, a technique called frequency hopping is used to maintain network operations, enabling coexistence with the other networks.
When a wireless network is constructed in a room, receivers form a multipath environment to receive an overlap of a direct wave and a plurality of reflected waves or delay waves. Multipath generates a delay distortion (or frequency selective fading) to cause a communication error. Further, a delay distortion causes inter symbol interference.
A major countermeasure against delay distortion can be a multi-carrier transmission system. The multi-carrier transmission system transmits data by dividing it into a plurality of carriers having different frequencies. Each carrier uses a narrow band and is hardly subject to frequency selective fading.
For example, the OFDM (Orthogonal Frequency Division Multiplexing) system, one of multi-carrier transmission systems, configures a frequency of each carrier so that the carriers become orthogonal to each other in a symbol region. During information transmission, the system converts serially transmitted information into parallel information at a symbol frequency lower than the information transmission rate. The system allocates a plurality of pieces of output data to each carrier, modulates the amplitude and the phase for each carrier, and performs the inverse FFT for the carriers. In this manner, the system converts the carriers into signals along the time-domain by maintaining the orthogonality of each carrier along the frequency-domain. The reception occurs in the reverse order of the transmission. The system performs the FFT to convert signals along the time-domain into those along the frequency-domain and demodulates the carriers in accordance with the modulation of each carrier. The system performs parallel-serial conversion to reproduce the information that was originally transmitted in the serial signals.
The OFDM modulation system is adopted as a wireless LAN standard in the IEEE802.11a/b, for example. The IEEE802.15.3 standardization is also in progress for the UWB communication system using the OFDM modulation system in addition to the Direct Sequence-Ultra-Wide Band (DS-UWB) system and the impulse-UWB system. The DS-UWB system increases spread speeds of DS information signals to the utmost limit. The impulse-UWB system uses impulse signal sequences having very short frequencies of several hundred picoseconds to configure information signals for transmission and reception. The OFDM_UWB communication system investigates an OFDM modulation that performs frequency hopping (FH) for a frequency band of 3.1 through 4.8 GHz into three sub-bands each comprising 528 MHz bandwidths and uses IFFT/FFT with frequency band comprising 128 points (e.g., see non-patent document 1).
On the other hand, the UWB communication uses ultra narrow pulses to provide high time resolution. This property can be applied to ranging for radar and positioning. In particular, the latest UWB communication can provide both high-speed data transmission over 100 Mbps and the intrinsic ranging function at the same time (e.g., see patent document 1).
In the future, it is expected that WPAN (Wireless Personal Access Network) for near field communication represented by the UWB communication is installed in all household electrical goods and CE (Consumer Electronics) devices. Therefore, in addition to the high-speed data transmission, it is considered to use position information based on the ranging, e.g., provide wireless added values such as navigation and Near Field Communication (NFC). It may be desirable to provide not only the high-speed data transmission, but also the ranging function.
For example, the UWB communication standardization in IEEE802.15.3 includes the UWB ranging technology as well as the OFDM modulation system (e.g., see non-patent document 1).
Generally, a transmission system using OFDM uses a frequency region for channel characteristic estimation that is performed reception and decoding. In this case, the receiver receives signals along the time-domain for data transmission and performs FFT to convert the received signals into signals along the frequency-domain. The receiver then extracts OFDM sub-carriers as mentioned above. Signals after FFT can be used for the channel characteristic estimation. However, the ranging requires a channel estimation value for time response, thus necessitating inverse FFT (see FIGS. 3 and 4).
The inverse FFT for ranging is a process independent of the intrinsic data transmission and therefore increases the receiver's load. When the FFT function for transmission is also used for the channel characteristic estimation, the FFT uses the large number of points similarly to ordinary reception, increasing the process amount.
For example, let us assume that a broadband channel characteristic estimation is performed using all bands in an OFDM modulation communication system that performs multiband frequency hopping. In such case, the FFT needs to use the larger number of points, complicating the apparatus. When an OFDM modulation system operates in 3-band mode where each frequency band comprises 128 points, it just needs to use the 128 points FFT for ordinary data reception. If the ranging uses all bands, it is necessary to use the 384 points FFT, three times larger than the size for the data transmission.
[Patent document]
    Japanese Translation of Unexamined PCT Appln. 2002-517001.[Non-patent document]            IEEE802.15.3a TI Document <URL:http://grouper.ieee.org/groups/802/15/pub/2003/May03 filename:03142r2P802-15_TI-CFP-Document.doc>        